In the production of gears, especially bevel gears, two types of processes are commonly employed, face milling (intermittent indexing) processes and face hobbing (continuous indexing) processes.
In face milling processes, tooth slots are formed individually in succession by feeding a rotating tool into a workpiece to a predetermined depth, withdrawing the tool, and indexing the workpiece to the next tooth slot position. The steps of feeding, withdrawing and indexing are repeated until all tooth slots are formed. This type of face milling process is known as a non-generating process. The profile shape of a tooth on a workpiece is produced directly from the profile shape on the tool.
Alternative to non-generated face milling, a face milling generating process may be performed wherein once the tool is fed to a predetermined depth, the tool and workpiece are then rolled together in a predetermined relative rolling motion, known as the generating roll, as though the workpiece were rotating in mesh with a theoretical generating gear, the teeth of the theoretical generating gear being represented by the stock removing surfaces of the tool. The profile shape of the tooth is formed by relative motion of the tool and workpiece during the generating roll. The steps of feeding, rolling, withdrawing and indexing are repeated for each tooth slot until all tooth slots are formed.
In face hobbing processes (non-generated or generated), the tool and workpiece rotate in a timed relationship and the tool is fed to depth thereby forming all tooth slots in a single plunge of the tool. After full depth is reached, a generating roll may be performed.
Since there is no indexing rotation of the workpiece during a face milling process, the rotating tool produces a circular lengthwise tooth curvature on the workpiece. In contrast, due to the additional rotation of the workpiece during face hobbing, a lengthwise tooth curvature having the form of an epicycloid is produced. Therefore, in a finishing process such as grinding, a circular (e.g. cup-shaped) grinding wheel is suitable for finishing the teeth of a workpiece that have been formed by a non-generated or generated face milling process but is usually not considered suitable for finishing face hobbed (epicycloid) teeth. Face hobbed gears are generally finished by a lapping process wherein the members of a gear pair (i.e. ring gear and pinion) are rotated together in the presence of an abrasive compound or slurry.
As mentioned above, face milled bevel gears may undergo a finishing process subsequent to cutting. Usually the finishing process is grinding utilizing a cup shaped grinding wheel. The type of grinding process (i.e. generating or non-generating) is usually the same as the type of face milling process utilized in cutting the gear. If desired during the grinding process, the axis of the grinding wheel may be revolved about an eccentric axis spaced from and parallel to the axis of the grinding wheel in manner as disclosed in U.S. Pat. No. 3,127,709. Such a method effectively provides an oscillating motion of the grinding wheel between adjacent tooth surfaces thereby enhancing the access of coolant to the grinding zone and the removal of chips therefrom.
Grinding of bevel and hypoid gears may cause a tooth surface roughness structure with lines of fine scratches that extend parallel to the root portion of the tooth. Imperfections of those lines often repeat on successive teeth which leads to a magnification of the amplitudes of the tooth mesh frequency and their higher harmonics. This phenomenon is known in grinding and in some applications has led to additional finishing operations, such as honing in cylindrical gears or short term lapping of bevel gears for example.